When high-quality images are needed, such as for diagnostic imaging applications, photosensitive media, such as film, paper, and other photosensitized substrates have marked advantages over many other types of substrates. In order to tap these advantages for images that are obtained or stored as digital data, a number of electronic printers have been developed.
One approach for exposure of a digital image onto a photosensitive medium uses a two-dimensional spatial light modulator, such as a liquid crystal device (LCD) or digital micromirror device (DMD). These devices expose a complete image frame at a time. Other printers employ linear light modulators with an array of light-emitting exposure elements, such for example as a micro light valve array (MLVA) using lead lanthanum zirconate titanate (PLZT) light valves (sold for example as the model QSS-2711 Digital Lab System manufactured by Noritsu Koki Co., located in Wakayama, Japan). This type of printer provides scanning movement of a linear array of exposure sources with respect to the surface of a photosensitized substrate. Alternate linear array exposure sources includes light emitting diode (LED) arrays. LEDs offer advantages such as low energy requirements, compact packaging, long life, relatively low cost, component durability and resistance to shock and vibration, and very good color performance and power output levels. Still other types of printers have adapted CRT devices as exposure sources. Printers employing lasers have also been developed to provide “flying spot” devices using a laser and a spinning polygon scanner, in similar fashion as in desktop laser printers.
Any type of imaging method for photosensitive media provides exposure radiation to which the media responds in a controlled manner. As is well-known, exposure energy is a factor of both the intensity of light radiation and the amount of time the radiation is applied, expressed in the familiar equation:E=It   (1)where I corresponds to the intensity and t corresponds to exposure duration.
Where a complete image frame is exposed in one operation, such as is done in conventional optical exposure and with two-dimensional spatial light modulators such as LCDs, control of the time factor t is relatively straightforward. For electronic images, each pixel in the image can be exposed during the same time interval. However, where only a portion of the image is exposed at a time, such as with the polygon scanner or linear light modulator approach, control of exposure time t becomes more complex. With these printers, a scanning sequence must scan the exposure beam or beams across the media at a constant rate and intensity for each pixel in order to maintain uniformity in the output image.
In the flying-spot imaging apparatus used in laser printers, the spinning polygon and cooperating optical system are designed to control these factors to provide substantially uniform exposure to each pixel in the image. One solution, as disclosed in U.S. Pat. No. 4,835,545 (Mager et al.) adjusts the intensity of the exposing laser based on the sensed velocity of a photosensitive medium as it is being moved past a laser imager scan line. U.S. Pat. No. 4,620,200 (Fukai) discloses another flying spot apparatus which measures the speed of the scanning spot and makes corrections in the intensity of the beam based on the speed. Both of these references, however, are high cost apparatuses.
Linear array printers present a different set of difficulties. With a linear scanner printing system, a precision mechanical arrangement is needed to provide mechanical movement of the printhead relative to the photosensitive medium. As is emphasized in commonly-assigned U.S. Pat. No. 4,475,115 (Garbe et al.), it is considered to be impractical and expensive to implement a scanning mechanism that, by itself, provides the required precision needed for transporting a photosensitive media past a linear array of exposure sources without some amount of error, which results in banding or other motion-related non-uniformities in the output image. Additional compensation is required from timing control circuitry.
Facing this same problem for image sensing applications, input optical scanning apparatus have used a number of techniques for scanning a multipixel linear sensor across a platen. For example, U.S. Pat. No. 6,037,584 (Johnson et al.) discloses a mechanical system with improved motion accuracy, in which an exposure control system varies exposure time for each pixel to compensate for speed variations and varies the gain applied to the sensed signal based on exposure time variations. Similarly, U.S. Pat. No. 6,576,883 (McCoy) discloses exposure control for an optical scanner, using non-linear gain compensation for exposure time variation. Both U.S. Pat. Nos. 6,037,584 and 6,576,883 provide useful techniques for input optical scanning using a linear sensor, however, the challenges faced in printing by exposure from an array of light sources are considerably more formidable, due to higher resolution and positional accuracy requirements and to response sensitivity characteristics of the photosensitive medium itself. Relatively considered, the accuracy requirements of optical printing are an order of magnitude higher than those of ink jet printing.
Laser thermal printing apparatus have employed various techniques for scanning a high-precision imaging printhead across the surface of a photosensitive medium with the timing accuracy necessary for accurate exposure. For example, the Kodak Approval Digital Proofing System uses a configuration in which a multichannel printhead travels in a path parallel to the axis of a rotating vacuum drum, with the substrate held in place on the vacuum drum. This arrangement is suitable for the large-format prepress imaging environment; however the size, complexity, and expense of a rotating vacuum drum prevents the use of this type of solution in a low-cost desktop optical printing system.
U.S. Pat. No. 6,422,682 (Kaneko et al.) discloses a carriage-mounted scanner that can be used interchangeably for ink jet printing or for optical scanning. The apparatus of U.S. Pat. No. 6,422,682 provides positional precision using an encoder strip and accumulation time measurement. This mechanism compensates for inherent inaccuracies in motor and drive mechanics for a carriage-mounted scanning head. Again, however, while corrective measures applied to the apparatus design compensate for tolerance errors in both position and timing, the end-result is suitable only for optical sensing or for ink jet droplet placement. Relatively considered, the accuracy requirements of optical printing are an order of magnitude higher than those of ink jet printing. The challenge of high-resolution optical printing using a carriage-mounted printhead, are not addressed in either U.S. Pat. Nos. 6,037,584; 6,576,883; or U.S. Pat. No. 6,422,682 and not satisfactorily met using solutions that have worked for prepress imaging systems.
It is instructive to observe that conventional ink jet printers have successfully employed carriage-mount designs using an encoder strip, as is employed in U.S. Pat. No. 6,422,682. The use of an encoder strip helps to compensate for velocity variations as the carriage reciprocates back and forth across the print platen. It must be emphasized that position, rather than dwell time, is the key consideration for placement of ink jet droplets onto a substrate. For example, the ink jet printhead can be controlled to eject drops at different rates during ramp-up and ramp-down as the printer carriage moves from one end of the print platen to the other. That is, in a carriage-mounted ink jet printhead, variations in printhead speed over the carriage length are compensated for by sensing markings on the encoder strip. Again, however, while conventional use of an encoder strip provides sufficient accuracy for ink droplet placement at the needed resolution, optical imaging requires significantly finer resolution. Moreover, by comparison, ink jet printing using a linear printhead of nozzles is inherently more “forgiving” in other ways than is optical printing using a linear array of light sources. For example, an ink jet printhead can be passed over the same area of the print substrate multiple times, allowing various techniques for interleaving, feathering, and patterning compensation to be readily applied. Optical printheads do not enjoy this advantage. Moreover, the number of individual channels in a linear optical printhead must be kept low due to power dissipation in the printhead.
Thus, it can be seen that requirements for high-resolution accuracy and for compensation of velocity changes along the scanning head path constrain the design of optical printheads using linear exposure arrays. As with the device disclosed in U.S. Pat. No. 4,475,115, conventional design approaches strongly favor stationary mounting for a printhead using a linear array of exposure sources and scanning of the photosensitive medium relative to this stationary exposure array, rather than using a carriage-mounted printhead. This conventional approach, however, does not allow optical printhead design to benefit from some of the advantages of carriage mounting designs, including compact size (particularly when using LED arrays), reduced cost for lower manufacturing volumes, and improved throughput.